Generally it is known to use a pump for incompressible fluids, such as oil. Often such pumps are of the variable capacity vane type. Such pumps include a moveable pump ring, which allows the rotor eccentricity of the pump to be altered to vary the capacity of the pump.
Having the ability to alter the volumetric capacity of the pump to maintain a pressure is desirable in environments such as automotive lubrication or oil pumps, wherein the pump will be operated over a range of operating speeds. In such environments, to maintain an equilibrium pressure it is known to employ a feedback supply of the working fluid (e.g. lubricating oil) from the output of the pump to a control chamber adjacent the pump control ring or slide, the pressure in the control chamber acting to move the control ring, against a biasing force applied to the control ring from a return spring, to alter the capacity of the pump.
Typically, for such oils pumps that are operated by the engine of the vehicle, the pressure at the output of the pump increases as the operating speed of the pump increases, the increased pressure is applied to the control ring (or slide) to overcome the bias force of the return spring and to move the control ring to reduce the capacity of the pump, thus reducing the output volume and hence the pressure at the output of the pump.
As the pressure at the output of the pump drops when the operating speed of the pump decreases, the pressure applied to the control chamber adjacent the control ring (or slide) decreases. When the pressure applied to the control chamber adjacent the control ring decreases the bias force of the return spring moves the control ring to increase the capacity of the pump, raising the output volume and hence pressure of the pump. In this manner, an equilibrium pressure is obtained and/or maintained at the output of the pump.
Conventionally, the equilibrium pressure is selected to be a pressure that is acceptable for the expected operating (e.g., speed) range of the engine. Necessarily, the selected equilibrium pressure is a compromise because the engine operates over a generally very wide range of speeds. The equilibrium pressure is selected so the oil pump will operate acceptably (to supply sufficient oil to the engine) at lower operating speeds with a lower working fluid pressure than is required at higher engine operating speeds (to supply a greater amount of oil to the engine). To limit undue wear or other damage to the engine, the engine designers will generally select an equilibrium pressure for the pump which meets the worst case (high operating speed) conditions. When this is the case, generally, at lower speeds, the pump will be operating at a capacity greater than necessary for those speeds thereby wasting energy pumping the surplus, unnecessary, working fluid.
Accordingly, there remains a significant need to improve the performance characteristics of a variable capacity vane pump having at least two equilibrium pressures and providing for greater packaging flexibility while providing a more compact pump.